Human cell division is a highly coordinated set of events that ensures the proper transmission of genetic material (chromosomes) from one mother cell to two newly formed cells. Chromosome missegregation during cell division can lead to aneuploidy (an aberrant chromosomal number), which is a hallmark of most cancers and has been proposed to promote tumorigenesis. Critical to ensuring proper sister chromatid separation at the metaphase to anaphase transition is the multi-component spindle assembly checkpoint (SAC), which is activated when unattached kinetochores or nonproductive (monotelic, syntelic, and merotelic) attachments are sensed and functions to arrest cells in metaphase to give time to correct these deficiencies and generate proper microtubule-kinetochore attachments before proceeding with cell division. Interestingly, a functional SAC plays a role in the effectiveness of chemotherapeutic drugs like antimitotics (drugs that inhibit mitosis), which damage the mitotic spindle, activate the SAC, arrest cells in prometaphase and trigger apoptotic cell death. Because understanding the SAC is critical to understanding tumorigenesis and the response of tumor cells to antimitotic drugs, it has become an attractive area of research. Although the last 40 years of research has shed light on the SAC, we are far from elucidating the full complement of regulatory factors involved in this complex pathway and from understanding how misregulation of this pathway can lead to tumorigenesis and resistance to chemotherapeutic drugs like antimitotics. To address these issues, we recently performed a high-throughput small interfering RNA (siRNA) screen for novel regulators of the SAC. This approach yielded two novel cyclin dependent kinases (Cdk14 and Cdk15) and two dual specificity phosphatases (DUSP7 and DUSP12). Inactivation of these novel factors leads to SAC bypass in the presence of antimitotic drugs like Taxol, which normally activate the SAC and induce cell death. To our knowledge, these factors have not been previously linked to SAC functioning and have limited genetic and molecular characterization in general. Thus understanding how these factors regulate the SAC is important to understanding the SAC and more broadly cell division. We hypothesize that these factors are controlling the SAC through their phosphorylation and dephosphorylation activities. Thus, we will test this hypothesis by analyzing the function of these proteins during unperturbed cell divisions and in the presence of antimitotics that generate spindle damage and activate the spindle assembly checkpoint. We expect that our studies will identify and characterize new factors and pathways that regulate the SAC, that we will be able to map their roles in time and space, and that our contributions to understanding the mechanisms of cell division will impact future cancer studies and cancer patient quality of life and survival.